A team from the Medical Examiner’s office worked all night to identify the bodies, which were removed by Saturday morning, while officials notified parents. It could take at least two more days for investigators to finish combing over the crime scene.

“We’ve been doing everything we need to do to peel back the onion, layer by layer, and get more information,” Connecticut State Police Lt. Paul Vance said.

"Our investigators at the crime scene ... did produce some very good evidence in this investigation that our investigators will be able to use in, hopefully, painting the complete picture as to how - and more importantly why - this occurred.”

Police have already determined that gunman Adam Lanza, 20, was not buzzed into Sandy Hook, where he was once a student.

“He forced his way into the school,” Vance said. He did not provide specifics and said that broken windows at the school may have been shattered by police who responded to the emergency.

Several officials told NBC News that once Lanza was inside, he shot the 20 children at close range, then killed himself. His mother, Nancy Lanza, was found shot to death at her home in Newtown.

Law enforcement officials gave out conflicting details about what type of weapons Lanza had.

At one point, they told NBC News Lanza had four handguns while he stalked the halls of Sandy Hook, but that could not be confirmed. It appears he carried at least two 9mm handguns – and a rifle was also found at the scene.

NBC's Pete Williams reports that Connecticut school shooter Adam Lanza attempted to purchase a rifle earlier this week at a sporting goods store in Danbury, Conn.

Officials also told NBC News that Lanza unsuccessfully tried to buy a rifle at a Dick’s Sporting Goods store in Danbury three days before the slaughter, but later said they could not confirm the report, which was based on information from members of the public.

At a Saturday briefing, Connecticut State Police Lt. Paul Vance said officials were tracing every weapon connected to the shooter.

The motive for the mass killing was unknown, but officials told NBC's Pete Williams that they were investigating a report that someone had an "altercation" with four staff members at the school on Thursday – three of whom were killed the next day.

“They were wondering whether the children knew what was happening to them, whether they were afraid,” said Monsignor Robert Weiss of St. Rose of Lima Church, who met with the families overnight at a firehouse while workers from the medical examiner's office identified the bodies.

A state trooper has been assigned to the family of each victim so they can grieve in private, Vance said. “This is an extremely heart-breaking, difficult thing,” he said.

Police provided little information about the shooter’s state of mind. Lanza’s brother told police the gunman had a history of mental problems, though a classmate from Newtown High School recalled him as a generally happy person.

“We would hang out, and he was a good kid,” Joshua Milas, who had not seen Lanza in a few years, told The Associated Press. “He was probably one of the smartest kids I know. He was probably a genius.”

Lanza’s connection to the school was unclear. Law enforcement officials initially said his mother was a kindergarten teacher there, but the school superintendent told TODAY that she wasn’t a full-time staffer and may not have any connection to the school.

Police said their investigation into Lanza and how he carried out the horrendous crime was still in the early stages.

“We have a mountain of evidence, both physical and forensic,” Newtown Police Lt. George Sinko said. “Obviously we’re not going to leave any stone unturned.”

He said the entire town of 27,000 – a sleepy bedroom community some 60 miles from New York City and known for its good schools – was reeling.

“We never thought this would happen here,” Sinko said. “Our hearts are broken for the families of these victims.”

Even the young survivors -- ages 5 to 10 -- will be scarred. They spoke of huddling in corners of locked classrooms and closets with heroic teachers while shots rang out around them.

Sandy Hook Elementary principal Dawn Hochsprung died in the attack after reportedly running toward the gunfire to protect her students. TODAY's Erica Hill reports, and Savannah Guthrie talks with two men who knew her.

Robert Licata said his 6-year-old son was in class when the gunman burst in and shot the teacher.

"That's when my son grabbed a bunch of his friends and ran out the door," he told The Associated Press. "He was very brave."